1. Field of the Invention
The invention relates to an ophthalmic lens containing a Fresnel surface and a method for manufacturing same.
2. The Prior Art
Lenses and other articles manufactured at optical quality, have exacting demands for mold replication, high optical transmission and impact resistance. Injection molding of lenses requires edge gating so that the runner ends up remote from the lens surfaces. The paths from the gate to the edge points of the mold cavity are not symmetrical and therefore make it difficult to control the thermodynamics of the cooling melt flow. As lens cavities become thinner, straight injection molding techniques are unable to fill the mold without premature freeze-off. Accordingly, injection molding machines have been modified to enlarge the cavity during some phase of the injection cycle, in a so-called injection/compression process.
For ophthalmic lenses, plastic materials represent a safer, thinner and lightweight alternative. As the demand for thinner and lighter lenses increases, there is a greater need for materials and optical designs that have a higher index of refraction and better performance.
Microstructured surfaces can impart certain functionality to an ophthalmic lens. For example, an ophthalmic lens containing a Fresnel microstructure can be made thinner and lighter, than a non-structured lens with same power.
As described above, thin lenses cannot be made with conventional injection molding, because of issues relating to mold replication. Similarly, microstructured surfaces which are more challenging, cannot be replicated completely and precisely using a structured metal insert in an injection molding process.
One proposal to make a microstructured surface described in U.S. Pat. No. 4,146,306 uses thin sheets of plastic material such as cellulose acetate, Tenite, Vinylite, polystryrene or methyl methacrylate. The ridges which form the microstructured surface are made by engraving concentric grooves or by making a matrix, or mold, in the desired shape. Where the parts are molded separately, they can then be either cohesively or adhesively bound together by the application of a suitable translucent adhesive, or cement, as is known in the optical art. Another method of manufacture would be to mold the first layer with the ridges and then to place such layer in another mold and pour the molten material into the mold cavity. The proposed methods are costly in that they involve multiple steps such as forming the films, engraving complementary ridges and grooves in two or more different films, and then adhering the various films together.
Injection molding, which has a fixed mold temperature below a material's glass transition temperature (Tg), can't replicate surface microstructure with high fidelity. FIG. 1A shows a rounded ridge of a Fresnel microstructure. In other words, the molten resin freezes off thereby forming a rounded tip before it reaches the corner of the ridge. The microstructure mold has increased surface area compared to a smooth mold surface. Since the mold temperature is lower than the Tg, the resin is subject to solidification as it rolls along the mold microstructure surface. In the corner of the ridge, the resin is rolling along two surfaces that are converging at the corner of the ridge. The rolling action causes the resin to cool to a rounded shape before it reaches the sharp corner, as can be seen in FIG. 1A. Because of process limitation, thermodynamic considerations and polymer overheating, the resin cannot be heated sufficiently to reach the corner and still produce a lens with acceptable optical qualities.
Injection-Embossing molding, which inject melt into mold at the temperature above material's Tg, and eject the part at the temperature below the Tg, can replicate microstructure with high fidelity. FIG. 1B shows a well replicated Fresnel microstructure.
The Injection-Embossing molding technique has successfully molded polycarbonate Fresnel lens with high structure replication quality. However, compared to injection molding, it has a longer cycle time (15 minutes for current processing condition). Also, it needs a specific thermolator, with both heating and cooling functions, to change mold temperature. More particularly, some molds are equipped with thermal control channels that circulate fluid through the mold near the mold inserts. A thermolator is a device which can quickly alternate between heated fluid and cooling fluid. During the injection molding cycle, heated fluid is circulated to raise the temperature of the mold inserts, e.g. above the Tg, to allow the resin to flow longer and completely fill the mold. Then the cooling fluid is circulated to solidify the resin by bringing it below its Tg.
Furthermore, the Injection-Embossing molding technique has been successfully applied to polycarbonate (PC) and poly(methyl methacrylate) (PMMA). However, it does not address problem associated with molding materials having a Tg below room temperature, such as thermoplastic polyurethane (TPU) and ethylene/methacrylic acid (E/MAA) copolymer.
Additional methods have been proposed to incorporate functional layers in lenses. For example, U.S. Pat. No. 6,367,930 discloses a multi-ply approach where a photochromic layer is inserted in to a mold in a so-called film insert molding method. In an alternate embodiment, polycarbonate is injected in to a mold, followed by a photochromic TPU in a so-called over-mold process. The patent does not mention the incorporation of a Fresnel lens in to a lens.
Accordingly, there is a need for an ophthalmic lens containing a Fresnel lens, with all of its intrinsic properties and additionally a surface layer containing a microstructure with high fidelity.